Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles with this free sleep calculator that runs right in your browser.
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Result
Fill in the fields to see your result.
Based on 90-minute sleep cycles plus ~15 minutes to fall asleep.
Estimate only — not medical or nutritional advice. Individual needs vary; consult a professional before making big changes.
How to use this calculator
- Choose whether you're setting a wake time or a bedtime.
- Enter the time.
- Read suggested times for 4–6 sleep cycles.
How it works
Sleep runs in ~90-minute cycles; waking at the end of a cycle feels more refreshing. The tool adds ~15 minutes to fall asleep.
Worked example
Wake at 7:00 am. Good bedtimes: ~9:45 pm (6 cycles) or ~11:15 pm (5 cycles).
Tips
- Estimate only — not medical or nutritional advice. Individual needs vary.
Frequently asked questions
Is 90 minutes exact?
It's an average — real cycles vary 70–120 minutes — so treat the times as a guide.
Privacy?
Runs entirely in your browser. Your inputs are not saved or transmitted.
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Cite this tool
Zerdly. (2026). Sleep Calculator. Retrieved from https://www.zerdly.com/tools/sleep-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.